1. Technical Field
The present invention provides the chemical and enzyme-catalyzed synthesis of melanins. The synthesized melanins inhibit the binding of viruses to animal cells, prevent a virus from infecting the cells of its host, and prevent the spread of viral infections from person-to-person.
2. Background Information
Melanin is a general term for a group of high molecular weight black and brown pigments that arise from the oxidation and polymerization of polyphenols and aromatic amino acids (Prota, G., Melanins and Melanogenesis. 1992, San Diego: Academic Press, Inc. 1-290; Riley, P. A., Int. J. Biochem. Cell Biol. (1997) 29(11):1235-1239). Melanins are normal constituents of animal and plant tissues and are also present in many microorganisms. Melanins contribute to the coloration of the skin and hair of animals, the seeds, berries, flowers, and leaves of plants and to subcellular structures in certain bacteria and fungi. Despite many years of careful study, the precise chemical structures of natural melanins are virtually unknown. Structural elucidation has been hampered because of four main factors: the low molecular weight phenolic precursors of melanins are highly diverse; the oligomeric polyphenol units of natural melanins contain substituents that exist in several different oxidation states and as multiple structural isomers; melanins are typically isolated as heterogeneous mixtures; and melanins are resistant to analytical approaches based on chemical degradation. The potential for variability in melanin structure is so great that it may well be that no two melanin pigments are absolutely identical.
Melanins are typically identified by their unique physicochemical properties including their solubility characteristics, their tendency to aggregate in the presence of metal ions, their characteristic adsorption spectra in the UV and infrared regions, their ability to act as semi-conductors, their decolorization by strong oxidizing agents, their antioxidant and radioprotective properties, and their resistance to hydrolysis under strongly acidic and strongly basic conditions. Natural and enzymatically synthesized melanins exhibit strong absorption throughout the UV and visible regions. Published spectra are characterized by a monotonic increase in the absorbance with decreasing wavelengths with a barely detectable shoulder between 290 and 320 nm. The infrared spectra of melanins show absorption bands expected for an aromatic or a heteroaromatic system of OH, NH, and conjugated carbonyl groups. Recent analyses of a natural melanin from Sepia officinalis using a combination of scanning electron microscopy, atomic force microscopy, x-ray scattering, and mass spectrometry are consistent with the notion that natural melanins are comprised of small oligomeric units that aggregate to form stable filaments, particles (5-200 nm in diameter) and aggregates of the particles (Clancy, C. M. et al., Biochemistry (2001) 40 (44):13353-13360). These data suggest that the molecular constituents of melanin oligomers are made up of 4 to 8 monomer units. The oligomers have masses in the range of 500 to 1500 amu. Even after extensive investigations of this well-known natural melanin using the most sophisticated analytical approaches available, neither the precise structures of the melanin polymer nor that of the oligomeric constituents have been ascertained. Regardless of the source, it is unlikely that natural melanins are derived from unique well-defined building blocks.
The bulk of the scientific literature on melanin structure and function pertains to the mammalian melanins. Considerably less is known about the plant-derived melanins even though they are an abundant constituent of the biomass. The biosynthesis of plant melanins is catalyzed by a family of Type III polyketide synthetases (e.g., chalcone synthetase) which catalyze the first committed step in the synthesis of a large number of biologically important flavonoids and phytoalexins. The PKS III enzymes utilize a starter CoA-ester (e.g., p-coumaroyl-CoA) from the phenylpropanoid pathway and catalyze three sequential condensation reactions with an extender CoA-ester (e.g., malonylCoA). This is followed by a ring closure, and an aromatization to form chalcones (e.g., naringenin). Many different aromatic phenolics can be made by this mechanism depending on which polyketide synthetase is present and which of the many possible starter and extender CoA-esters are available for incorporation. The aromatic phenols and polyphenols that are the products of the polyketide pathway in a particular plant are oxidized to quinones by the enzyme polyphenol oxidase. The quinones are then joined together by free radical polymerization reactions to form oligomers that contain a mixture of quinone, hydroquinone, and semi-quinone functional groups. The oligomers associate to form stable, non-covalent complexes that take the form of particles, aggregates of particles, and fibers and exhibit the distinctive physical and chemical characteristics of the natural melanins.
Traditionally, melanins have been isolated from plants as heterologous, crude or purified preparations. Banister et al. (U.S. Pat. No. 6,303,106) describes the enzymatic production of melanins from phenolic compounds and an enzyme selected from tyrosinase, laccase and peroxidase. The produced melanins are used in the production of cosmetics.
The published literature concerning the use of plant-derived melanins as therapeutic agents is minimal. However, there have been a number of publications describing the isolation and characterization of melanins from fermented black tea (Sava, V.M. et al., Food Chemistry (2001) 73: 177-184) and from grapes (Novikov, D.A. et al., Radiats. Biol. Radioecol. (2001) 41(6):664- 670; Avramidis, N., et al., Arzneimittelforschung. (1998) 48(7): 764-771). These plant-derived melanins have been shown to possess a number of interesting pharmacological properties including immunomodulatory activity (A vramidis, 1998; Sava, V.M. et al., Food Research International (2001) 34: 337-343), photoprotective activity (Novikov, 2001), metal binding properties that make them useful as image enhancers in MRI (Hung, Y.C. et al., J. Ethnopharmacol. (2002) 79(1): 75-79), and the ability to protect animals from oxidant-induced tissue damage (Hung, Y.C. et al., Life Sci. (2003) 72(9): 1061-1071). Montefiori (U.S. Pat. No. 5,057,325) describes inhibition of human immunodeficiency virus (HIV) by chemically synthesized melanins, preferably melanins containing subunits of L-Dopa.
Synthetic melanins have been shown to have activity against HIV infections in in vitro cell culture models (Novikov, 2001; Garger, U.S. Pat. No. 6,440,691). The intrathecal administration of synthetic and natural melanins has been proposed as a treatment for certain neurodegenerative diseases, however evidence for their effectiveness has yet to be presented (Berliner, U.S. Pat. No. 5,817,631).
Crude and purified preparations of plant-derived melanins prevent viral infections by interfering with early steps in the infectious process namely the attachment of a virus to the epithelial cell membranes of a potential host and/or the fusion of the viral membrane with that of the host cell (Holmes, E. W. and Thompson, K. D., FASEB 1.19, (2005) Abstract No. 323.5, 2005). Host cells are protected from infection after only a brief period of melanin treatment and the protection afforded by the treatment persists for hours. Plant melanins are nontoxic to animal cells in vitro and cause no adverse effects following intravaginal administration in mice. Such properties make them ideal for the prevention of viral infections for which the portal of entry is the epithelium of the host.
Melanins have been chemically synthesized from hydroxyphenyl-containing precursors, mostly focused on catecholamines and catecholamine derivatives (e.g. DOPA, Dopamine, etc). Quinones are generated at basic pH spontaneously, followed by polymerization. However, the resulting polymers are often found to be poorly soluble in biocompatible solvents, and hence, are not likely to be useful as pharmaceuticals. Another disadvantage of many of the synthetic polymers of hydroxyphenyl compounds is that data documenting the scopes and potencies of their antimicrobial activities is either limited or unavailable.
It would be desirable to synthetically produce melanins in high yields under reproducible conditions. Further, it would be desirable to produce novel melanins, especially water-soluble melanins, with antiviral activity.